> On Sat Apr 29 02:23:10 UTC 2017, Richard Eisenberg wrote: > > I'm not quite sure what a restriction on (~) might be,
Thanks Richard, I was thinking that FunDeps are restricted to bare type vars. I can't write either of these: > class C a b c | a -> (b, c) -- per my O.P. (~) > class C a b c | a -> (b c) So it's the one place where type vars `b c` like this: > class C a b c | a -> b c doesn't mean applying `b` to `c`. > but (~) is effectively declared as > > > class a ~ b | a -> b, b -> a > > So I agree with your observations. Aha! So I could equivalently go: > class EqC a b | a -> b, b -> a > > class (EqC a (b, c)) => C a b c where ... > -- needs FlexibleInstances And that does indeed work equivalently to the (~). Again, I haven't put FunDeps on class `C`. So should I reasonably have known that a superclass constraint with FunDeps on the superclass induces FunDeps on the sub-class without explicitly needing to declare so? (I'm not complaining, more surprised/impressed.) AntC > > On Apr 27, 2017, at 8:14 PM, Anthony Clayden > > <anthony_clay...@clear.net.nz> wrote: > > The docos say [User Guide 10.14.1. on Equality > > Constraints] > >> Equality constraints can also appear in class and > > instance contexts. > >> The former enable a simple translation of programs > >> using functional dependencies into programs using > > family synonyms instead. > > > http://downloads.haskell.org/~ghc/8.0.2/docs/html/users_guide/glasgow_exts.html#equality-constraints > > > > And the forms of constraint seem quite sophisticated. > > I was surprised (pleased) I could do this: > > > > {-# LANGUAGE MultiParamTypeClasses, TypeFamilies, > > FlexibleInstances #-} > > > > type family F a > > > > class (F a ~ (b, c) ) => C a b c where -- (b c) > > !! f1 :: a -> b > > f2 :: a -> c > > > > Uses of `f1` happily improve the type for `b`. > > Uses of `f2` happily improve the type for `c`. > > > > But I didn't declare a Functional Dependency. > > (It seems to do no harm if I add `| a -> b c`.) > > > > GHC's behaviour seems stronger than a "simple > > translation". It seems entirely equivalent to a FunDep. > > > > Or is there something I'm missing? > > (I could have overlapping instances, > > but only providing the equations for > > type family `F` are confluent.) > > > > Are there restrictions on the form of Equality > > Constraints to get them to behave as FunDeps? > > (It's not merely a bare typevar on one side.) > > > > AntC > > _______________________________________________ > > Glasgow-haskell-users mailing list > > Glasgow-haskell-users@haskell.org > > > http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users > _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users
